1. Field of the Invention
The present invention relates generally to highway guardrail systems and road barriers. More particularly, the invention relates to an improved end treatment for guardrail systems.
2. Description of the Related Art
Along most highways there are hazards that can be a substantial danger to drivers of automobiles if the automobiles were to leave the highway. To lessen the danger guardrails are provided. Semi-rigid W-beam guardrails are usually positioned alongside vehicular traffic routes, especially highways, for the purposes of preventing vehicles from colliding with fixed objects, other vehicles, or driving off the roadway. To this end, the guardrails should have sufficient integrity to prevent an impacting vehicle approaching from an angle relatively oblique to the length of the rail from breaking or tearing through their structure and leaving the roadway. Thus, a guardrail is provided to aid in redirecting an obliquely impacting vehicle back onto the roadway while at the same time dissipating its impact force in a controlled manner.
Guardrails must be installed, however, such that the upstream end, or terminal end of the guardrail facing the flow of traffic is not, itself, a hazard. Early guardrails had no proper termination at the ends, and it was not uncommon for impacting vehicles to become impaled on the raised end of such a guardrail causing intense deceleration of the vehicle and severe injury to the occupants. In some reported cases, the upstream end of guardrail penetrated directly into the occupant compartment of the vehicle fatally injuring the occupants.
Upon recognition of the problem of proper guardrail termination, guardrail designs have been developed employing box beams and W-beams that allow sloping of the end of a guardrail into the ground. These designs provide a ramp and help to eliminate spearing effects. However, while these end treatments have successfully reduced the danger of a vehicle being penetrated in a head-on collision, it has been discovered that they also tend to induce launching of the vehicle to the extent of becoming airborne for a considerable distance with the possibility of rollover.
A number of alternate designs have focused on reducing the dynamic buckling resistance of a raised end-type guardrail against substantially end-on impacts to reduce potential damage to impacting vehicles and injury severity to their occupants. While these end treatments help to reduce dangerous vaulting and roll over of vehicles, they do not provide a panacea.
Designs are also known in which sections of guardrail are curved away from the roadway to create an eccentric loading upon the sections during an end-on impact. The end piece of this design typically includes a customized nose piece which may consist of a fabricated structural steel lever nose surrounded by a vertical section of corrugated steel pipe. The lever nose is adapted to induce a moment near the upstream end of the guardrail sections upon an end-on impact, thereby facilitating desired buckling in the guardrail sections. These designs are also intended to facilitate "gating" of an impacting vehicle through the guardrail structure from the roadway side of the guardrail to the opposite side of the rail. However, they have proven costly. They are also rather complicated and in actual field installation require careful attention to installation details.
Another safety treatment consists of overlapped guardrail sections that have a series of closely spaced slots. The guardrail segments are attached by bolts extending through the slots. When a vehicle impacts the end of this barrier, the bolts are forced to tear through the guardrail sections from one slot to the next. As a result, the guardrail segments are cut into several long ribbons as an impacting vehicle is decelerated. This safety treatment appears too costly for wide spread implementation as a guardrail end treatment.
A further alternate end treatment is constructed from a series of break away steel guardrail posts and fragile plastic containers containing sandbags. Impacting vehicles are decelerated as the guardrail posts are broken and sand bags in the plastic containers are impacted. A cable is used to guide vehicles away from the guardrail during impact. This type system is very expensive, and has not gained wide acceptance.
A recent development is a terminal in which longitudinal slots are cut into a section of a W-beam rail to reduce the rail's dynamic buckling strength during end-on impacts. The terminal is typically installed on a parabolic flare, or outwardly curving configuration, away from the roadway. Cover plates are used to shield the slots and prevent extension of the slots and tearing of the rail during oblique impacts. The cover plates are sections of W-beam guardrail placed directly over the slots and are bolted to at least one end of the slotted section of guardrail.
The design normally includes a groundline cable to facilitate fracture of support posts and help prevent bending and rotation of the posts prior to breaking. The groundline cable is anchored near ground level to the support post at the most downstream end of the terminal, and extends upstream through boreholes in intermediate posts and is then anchored to the most upstream post. During an end-on collision, the groundline cable prevents rotation of the posts while the bore in each post sufficiently weakens the posts allowing them to break off upon engagement with the impacting vehicle. Additional posts have been used with this design to aid in redirection of obliquely impacting vehicles.
Although this last design meets current safety test criteria, construction details such as the cover plates, groundline cable and additional posts complicate installation and maintenance and have hindered its acceptance in the highway community. The cover plates reduce the ability of the guardrail to buckle and collapse, thereby lessening the rate of energy attenuation. The presence of unreinforced slots to reduce the rail's dynamic buckling resistance to end-on impacts may also function to reduce the rail's integrity during oblique impacts. Heavy or fast obliquely impacting vehicles might actually tear through the rail rather than being redirected back into traffic by it.
It is desirable to provide a guardrail design, having particular application as an end treatment, which addresses the problem of end-on impacts while providing sufficient integrity for the guardrail structure to reduce the problems associated with laterally impacting vehicles tending to tear through the structure. It would further be desirable to provide a guardrail design which assists in redirecting laterally impacting vehicles back toward the roadway.